Acoustic isolator for downhole applications

ABSTRACT

A plurality of heavy mass irregularities attached to an inner wall of the drill collar attenuate waves traveling through the collar. The plurality of heavy mass irregularities are spaced and sized for the maximum attenuation of acoustic pulses in a predetermined frequency range. The mass irregularities may be rings firmly coupled to the outer surface of the collar. Alternatively, the mass irregularities may be rings firmly coupled to the outer collar surface by neck pieces, extending inwardly from the inner circumference of the ring. The mass irregularities may be made of steel or tungsten. In another preferred embodiment, the mass irregularities are asymmetrically coupled to an outer collar wall for providing preferential directional attenuation.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] The present application is a Continuation-in-Part of U.S. patentapplication Ser. No. 09/583,258 filed on May 31, 2000 that claimspriority from and is based upon U.S. Provisional Patent ApplicationSerial No. 60/137,388 filed on Jun. 3, 1999.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention pertains to logging while drillingapparatus and more particularly to acoustic logging while drillingapparatus and attenuation of acoustic pulses that travel parallel to thedirection of drilling.

[0004] 2. Related Prior Art

[0005] To obtain hydrocarbons such as oil and gas, wells or wellboresare drilled into the ground through hydrocarbon-bearing subsurfaceformations. Currently, much current drilling activity involves not onlyvertical wells but also drilling horizontal wells. In drilling,information from the well itself must be obtained. While seismic datahas provided information as to the area to drill and approximate depthof a pay zone, the seismic information can be not totally reliable atgreat depths. To support the data, information is obtained whiledrilling through logging while drilling or measuring while drilling(MWD) devices. Logging or measuring while drilling has been a procedurein use for many years. This procedure is preferred by drillers becauseit can be accomplished without having to stop drilling to log a hole.This is primarily due to the fact that logging an unfinished hole, priorto setting casing if necessary, can lead to washouts, damaging thedrilling work that has already been done. This can stall the completionof the well and delay production. Further, this information can beuseful while the well is being drilled to make direction changesimmediately.

[0006] Advances in the MWD measurements and drill bit steering systemsplaced in the drill string enable drilling of the horizontal boreholeswith enhanced efficiency and greater success. Recently, horizontalboreholes, extending several thousand meters (“extended reach”boreholes), have been drilled to access hydrocarbon reserves atreservoir flanks and to develop satellite fields from existing offshoreplatforms. Even more recently, attempts have been made to drillboreholes corresponding to three-dimensional borehole profiles. Suchborehole profiles often include several builds and turns along the drillpath. Such three dimensional borehole profiles allow hydrocarbonrecovery from multiple formations and allow optimal placement ofwellbores in geologically intricate formations.

[0007] Hydrocarbon recovery can be maximized by drilling the horizontaland complex wells along optimal locations within thehydrocarbon-producing formations. Crucial to the success of these wellsis establishing reliable stratigraphic position control while landingthe well into the target formation and properly navigating the drill bitthrough the formation during drilling. In order to achieve such wellprofiles, it is important to determine the true location of the drillbit relative to the formation bed boundaries and boundaries between thevarious fluids, such as the oil, gas and water. Lack of such informationcan lead to severe “dogleg” paths along the borehole resulting from holeor drill path corrections to find or to reenter the pay zones. Such wellprofiles usually limit the horizontal reach and the final well lengthexposed to the reservoir. Optimization of the borehole location withinthe formation also can have a substantial impact on maximizingproduction rates and minimizing gas and water coning problems. Steeringefficiency and geological positioning are considered in the industryamong the greatest limitations of the current drilling systems fordrilling horizontal and complex wells. Availability of relativelyprecise three-dimensional subsurface seismic maps, location of thedrilling assembly relative to the bed boundaries of the formation aroundthe drilling assembly can greatly enhance the chances of drillingboreholes for maximum recovery. Prior art down hole devices lack inproviding such information during drilling of the boreholes.

[0008] Modern directional drilling systems usually employ a drill stringhaving a drill bit at the bottom that is rotated by a drill motor(commonly referred to as the “mud motor”). A plurality of sensors andMWD devices are placed in close proximity to the drill bit to measurecertain drilling, borehole and formation evaluation parameters. Suchparameters are then utilized to navigate the drill bit along a desireddrill path. Typically, sensors for measuring downhole temperature andpressure, azimuth and inclination measuring devices and a formationresistivity measuring device are employed to determine the drill stringand borehole-related parameters. The resistivity measurements are usedto determine the presence of hydrocarbons against water around and/or ashort distance in front of the drill bit. Resistivity measurements aremost commonly utilized to navigate the drill bit. However, the depth ofinvestigation of the resistivity devices usually extends only two tothree meters and resistivity measurements do not provide bed boundaryinformation relative to the downhole subassembly. Furthermore, thelocation of the resistivity device is determined by some depth measuringapparatus deployed on the surface which has a margin of error frequentlygreater than the depth of investigation of the resistivity devices.Thus, it is desirable to have a downhole system which can accurately mapthe bed boundaries around the downhole subassembly so that the drillstring may be steered to obtain optimal borehole trajectories.

[0009] The relative position uncertainty of the wellbore being drilledand the critical near-wellbore bed boundary or contact is defined by theaccuracy of the MWD directional survey tools and the formation dipuncertainty. MWD tools may be deployed to measure the earth's gravityand magnetic field to determine the inclination and azimuth. Knowledgeof the course and position of the wellbore depends entirely on these twoangles. Under normal conditions, the inclination measurement accuracy isapproximately plus or minus two tenths of a degree. Such an errortranslates into a target location uncertainty of about three meters perone thousand meters along the borehole. Additionally, dip ratevariations of several degrees are common. The optimal placement of theborehole is thus very difficult to obtain based on the currentlyavailable MWD measurements, particularly in thin pay zones, dippingformations and complex wellbore designs.

[0010] Until recently, logging while drilling has been limited toresistivity logs, gamma logs, neutron logs and other non-acoustic logssince acoustic noise caused by drilling and acoustic pulses travelingupstring from the transmitter has presented problems in accuratedetection and delineation. These problems cannot be easily isolated byarrival time since the acoustic pulses are generated and detectedcontinuously. Recently, the use of acoustic sensors having a relativelyshort spacing between the receivers and the transmitter to determine theformation bed boundaries around the downhole subassembly has been used.An essential element in determining the bed boundaries is thedetermination of the travel time of the reflection acoustic signals fromthe bed boundaries or other interface anomalies. A prior art proposalhas been to utilize estimates of the acoustic velocities obtained fromprior seismic data or offset wells. Such acoustic velocities are notvery precise because they are estimates of actual formation acousticvelocities. Also, since the depth measurements can be off by severalmeters from the true depth of the downhole subassembly, it is highlydesirable to utilize actual acoustic formation velocities determineddownhole during the drilling operations to locate bed boundariesrelative to the drill bit location in the wellbore.

[0011] Additionally, for acoustic or sonic sensor measurements, the mostsignificant noise source is acoustic signals traveling from the sourceto the receivers via the metallic tool housing and those travelingthrough the mud column surrounding the downhole subassembly (tube wavesand body waves). In some applications acoustic sensor designs are usedto achieve a certain amount of directivity of signals. A transmittercoupling scheme with signal processing method may be used for reducingthe effects of the tube wave and the body waves. Such methods, however,alone do not provide sufficient reduction of the tube and body waveeffects, especially due to strong direct coupling of the acousticsignals between the transmitters and their associated receivers.

[0012] Some United States patents representative of the current art indetermining subsurface formations are as follows.

[0013] U.S. Pat. No. 4,020,452, titled “Apparatus For Use inInvestigating Earth Formations”, issued to Jean-Claude Trouiller, etal., relates to an apparatus for mechanically filtering acoustic pulsesin a well logging tool. This apparatus includes of a substantially rigidmember having interruptions in the longitudinal continuity of themember. These interruptions provide tortuous paths for the passage ofacoustic energy along the member. A plurality of masses are periodicallyspaced along the interior of the member and are each mechanicallyintegral with opposite sides of the member at locations chosen to enablethe member and masses to cooperate as a mechanical filter. By so doing,the structure made of the member and masses will have good acousticdelay and attenuation characteristics as well as good mechanicalcharacteristics.

[0014] U.S. Pat. No. 5,043,952, titled “Monopole Transmitter For a SonicWell Tool”, issued to David C. Hoyle, et al., relates to a monopoletransmitter for a sonic tool which includes an axial tube, apiezoceramic cylinder surrounding the axial tube, an endcap disposed ateach end of and firmly contacting the cylinder, and an apparatus forholding the endcaps firmly against the axial tube. The endcaps firmlycontact the axial tube without simultaneously contacting an upperbulkhead. The apparatus may include spring washers disposed between thebulkhead and at least one endcap, or it may include a spring disposedbetween a nodal mount and each endcap. A nodal mounting tube may bedisposed around the axial tube, a ring being disposed at each end of thenodal mounting tube, each ring being disposed outside of the cylinderfor biasing the endcaps in tension against a ring thereby holding eachendcap firmly in contact against the axial tube.

[0015] U.S. Pat. No. 5,510,582, titled “Acoustic Attenuator, WellLogging Apparatus and Method of Well Logging”, issued to James R.Birchak, et al., relates to a sonic well tool for performing acousticinvestigations of subsurface geological formations penetrated by aborehole. The well tool generally includes a longitudinally extendingbody for positioning in the borehole. The tool also includes atransmitter supported by the body for transmitting acoustic energy and areceiver supported by the body for receiving acoustic energy. The toolincludes an acoustic attenuation section positioned on the body betweenthe transmitter and the receiver. This section includes one or morecavities defined by the body, inertial mass members positioned insidethe cavities in a suitable manner to form a gap between the wall of thecavity and the inertial mass members, and an acoustical attenuationfluid in the gap. The method for attenuating sonic waves generallyincludes transmitting a sonic wave from the transmitter to the tool,passing the sonic wave through the acoustic attenuation section, andreceiving attenuated wave at the receivers.

[0016] U.S. Pat. No. 5,036,945, titled “Sonic Well Tool TransmitterReceiver Array Including an Attenuation and Delay Apparatus”, issued toDavid C. Hoyle, et al., relates to a sonic well tool that includes atransmitter array having at least one monopole transmitter and at leastone dipole transmitter and a receiver array for receiving sonic pressurewave signals from a surrounding borehole formation. A first attenuationand delay apparatus is positioned above the receiver array and a secondattenuation and delay apparatus is positioned below the receiver arrayin the sonic well tool. The first attenuation and delay apparatusincludes an attenuation member comprising a plurality of interleavedrubber and metal like washers for attenuating compressional and flexuralwaves propagating along a metal center support rod to the receiver arrayand an inner housing comprising a bellows section having a corrugatedshape and a thin transverse dimension for delaying the propagation ofcompressional and flexural waves along the inner housing to the receiverarray. The second attenuation and delay apparatus includes a pluralityof mass loading rings surrounding the outer housing of the sonic welltool for attenuating the flexural waves propagating up the outer housingfrom a sonic transmitter ad a further inner housing including a furtherbellows section having a corrugated shape and a thin transversedimension for delaying the propagation of compressional and flexuralwaves up the tool, along the inner housing, to the receiver array. Thesonic well tool also includes a differential volume compensator forchanging the quantity of oil encapsulated in the sonic well tool inaccordance with changes in oil volume and changes in boreholetemperature and pressure. The receiver array includes a plurality ofhydrophone sets, each hydrophone set including at least one pair andpreferably two pair of hydrophones disposed in a cross section of thetool, one hydrophone of a pair being disposed opposite the otherhydrophone of the pair in the cross section.

[0017] U.S. patent application Ser. No. 09/201,988, now U.S. Pat. No.6,082,484 to Molz & Dubinsky, having the same assignee as the presentinvention discloses the use of a section of a drill collar that has aplurality of shaped cavities filled with oil. The passage of an acousticwave sets up a resonance of the fluid in the shaped cavity. Thefrequency of resonance depends upon the shape and size of the cavity andthe properties of the fluid in the cavity. In one embodiment of theinvention, the cavities are spherical. Another embodiment of theinvention uses cylindrical cavities with a piston restrained by a springwithin the cavity. Changing the spring constant provides additionalcontrol over the frequencies that are attenuated. The '988 applicationalso discloses the use of segmented isolators in which the drill collarsection is filled with layers of a composite material in which thelayers have a different density. The thicknesses of the individuallayers is selected to attenuate certain frequencies.

[0018] U.S. patent application Ser. No. 09/583,258 to Egerev et al,having the same assignee as the present application and the contents ofwhich are incorporated herein by reference, discloses a system andmethod for attenuation of acoustic waves that travel through a drillcollar in a logging while drilling operation. The system includes aplurality of heavy masses attached to an inner wall of the drill collar.The heavy masses constitute mass discontinuities that attenuate wavestraveling through the drill collar. In one embodiment of the invention,the mass discontinuities are rings and attachment is done by neckpieces. These neck pieces extend out from the outer circumference of therings and may be an original outer circumference of the ring that hasbeen milled down by cutting out portions of the ring. This allowssignificantly less than the entire outer circumference of the hangingrings to be in contact with the inner surface of the drill collar. Thus,the rings will more efficiently attenuate the vibrational force of theacoustic pulses coming in contact with the hanging ring. The pluralityof heavy hanging rings are spaced and sized for the maximum attenuationof acoustic pulses in a predetermined range, preferably in the range of10 khz to 20 khz. The system may include steel rings as the plurality ofheavy hanging rings. In an alternate embodiment, the plurality of heavyhanging rings may be a heavier, more dense material such as tungsten.The plurality may have as many as ten rings or as few as six, with eightbeing another possibility. The spacing of the rings may vary betweentwelve and fourteen centimeters, depending on the material used. In astill further embodiment, a pipe may be placed within the innercircumference of the rings to isolate the attenuation rings from theflow of drilling mud. The isolation pipe may be of any material,however, a material that is non-rigid that is less likely to conductvibrational forces is preferred. In another embodiment of the invention,the mass discontinuities are attached to the drill collar over asubstantial portion of their individual axial lengths. Such anarrangement acts as a low pass filter. When this mechanical arrangementis used with an electrical bandpass filter in the tool, high frequenciesare efficiently attenuated. In yet another embodiment of the invention,the attenuator section comprises a cylindrical body with sections ofdifferent inside and/or outside diameters to produce a ringed pipe: thesections of different diameter each have a characteristic passband and areject band for attenuation of signals.

[0019] The attenuator system of Egerev is expensive to fabricate anddifficult to maintain due to the multiple mass discontinuitiesincorporated on the inner wall of a drill collar. The erosive flow ofdrilling fluid in the inside of the collar can cause severe damage tothe isolators absent an internal sleeve. It would be desirable to havean attenuator system that is less expensive to fabricate and easier tomaintain.

SUMMARY OF THE INVENTION

[0020] The present invention provides a system and method forattenuation of acoustic waves that travel through a drill collar in alogging while drilling operation. The system includes a plurality ofheavy masses attached to an outer wall of the drill collar. The heavymasses constitute mass discontinuities that attenuate waves travelingthrough the drill collar. In one embodiment of the invention, the massdiscontinuities are rings and attachment is done by neck pieces. Theseneck pieces extend in from the inner circumference of the rings. Thisallows significantly less than the entire inner circumference of therings to be in contact with the outer surface of the drill collar. Thus,the rings will more efficiently attenuate the vibrational force of theacoustic pulses coming in contact with the ring. The plurality of heavyrings are spaced and sized for the maximum attenuation of acousticpulses in a predetermined range, preferably in the range of 10 khz to 20khz. The system may include steel rings as the plurality of heavy rings.In an alternate embodiment, the plurality of heavy rings may be aheavier, more dense material such as tungsten. The plurality may have asmany as ten rings or as few as six. The spacing of the rings may varybetween twelve and fourteen centimeters, depending on the material used.In another embodiment of the invention, the mass discontinuities areattached to the drill collar over a substantial portion of theirindividual axial lengths. Such an arrangement acts as a low pass filter.When this mechanical arrangement is used with an electrical bandpassfilter in the tool, high frequencies are efficiently attenuated.

[0021] In another preferred embodiment, a system for preferentialattenuation comprises a plurality of spaced-apart masses asymmetricallyattached to an adjacent outer wall of the collar.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] For detailed understanding of the present invention, referencesshould be made to the following detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings, inwhich like elements have been given like numerals and wherein:

[0023]FIG. 1 is an illustration of a drill system having a measuringwhile drilling device mounted in the drilling apparatus;

[0024]FIG. 2 illustrates raypaths of acoustic signals between thetransmitter and the receiver;

[0025]FIG. 3 is an illustration of an attenuation system for use on awell drilling collar;

[0026]FIG. 4 is a graphical representation illustrating the effects ofan increased number of attenuation elements of a system as thatillustrated in FIG. 1;

[0027]FIG. 5 is a graphical representation illustrating the effects ofincreasing the weight of attenuation elements of a system as thatillustrated in FIG. 1.;

[0028]FIG. 6 is a graphical representation illustrating the attenuationeffect of the system of FIG. 1;

[0029]FIGS. 7a and 7 b show a comparison of the invention of FIG. 2 withone in which the mass discontinuities are attached to the drill collarover a substantial length;

[0030]FIGS. 8a-8 c show alternate embodiments of the invention in whichattenuation is accomplished by means of recesses that produce massdiscontinuities in a body of the attenuator;

[0031]FIG. 9 shows a comparison of frequency spectra of attenuatorshaving different types of recesses having a fixed length;

[0032]FIG. 10 shows alternate embodiments of the invention in which thediameter of the attenuation sections is varied;

[0033] FIGS. 11 shows an alternate preferred embodiment using anarrangement of mass bodies attached to an external wall of a drillcollar; and

[0034]FIG. 12 shows an asymmetrical arrangement for a mass ring attachedto an external wall of a drill collar.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0035] The present invention provides a system and method forattenuating acoustic waves in a down hole tool that is being used toobtain information about subsurface formations, some of which arebelieved to be holding hydrocarbon deposits. FIG. 1 is a schematicillustration of the use of a Measurement-While-Drilling (MWD) apparatuswhile drilling a well. At the surface of the earth 5 a drilling rig 1 isused to drill a borehole 23 through subterranean formations 25 a, 25 b,25 c etc. Those versed in the art would know that a drillship or aplatform could be used to drill a borehole into subterranean formationscovered by a body of water. A drilling tubular 13, that could be made ofdrill pipes or coiled tubing is used to rotate a drillbit 17 at thebottom, the rotating action of the drillbit and axial pressure carvingout the borehole. When coiled tubing is used for the drilling tubular, adrilling motor (not shown) is used to impart the necessary rotary motionto the drillbit.

[0036] A variety of transducers are used downhole in a sensor assembly11. This sensor assembly makes measurements of properties of theformations through which the borehole is being drilled. These couldinclude electromagnetic, gamma ray, density, nuclear-magnetic resonance,and acoustic sensors. For illustrative purposes only, an acoustictransmitter array 31 and an acoustic receiver array 33 are indicated.Those versed in the art would recognize that other configurations of theacoustic transmitters and receivers could be used.

[0037] Turning now to FIG. 2, the transmitter 31 and the receiver 33 areshown inside the borehole 23. The annulus between the drilling tubular13 and the borehole 23 is filled with a drilling fluid. The fluid isconveyed down the borehole inside the drilling tubular to the drillbitand returns up the hole via the annulus. Excitation of the transmitterproduces acoustic signals. A portion of the signal, denoted by theraypath 43, is referred to as the direct arrival and travels through thetool to the receiver. The transmitter also produces an acoustic signalin the borehole fluid that enters into the formation. One portion of it,illustrated by the raypath 41 travels as a body wave through theformation and carries information about the formation that it traverses.The receiver also detects other signals, such as tube waves that involvea coupled wave between the fluid and the formation, Stoneley waves thatare surface waves in the fluid, and signals reflected from acousticreflectors within the formation.

[0038] In an MWD tool, as in wireline tools, the body wave 41 throughthe formation usually arrives before the tube wave and the Stoneleywave. However, in an MWD tool, the direct arrival 43 through the toolcommonly arrives before the desired signal component 41 that carriesinformation about the acoustic properties of the formation. In addition,the drillbit 17 itself is continuously generating acoustic signalstraveling through the drilling tubular 13. Consequently, it becomes verydifficult to determine a travel time for the formation body wave 41.

[0039] In order to attenuate the direct arrival 43, the tool a pulseattenuator 40 is located in tool 11 between transmitter 31 and anreceiver 33. Only one transmitter and receiver are illustrated fordemonstration. In practice, there may be several receivers andtransmitters and the present invention operates with any arrangement,the only requirement is that attenuator 40 be located between thetransmitter and the receiver.

[0040] In one embodiment of the invention, the acoustic isolator isbased upon an array of mass rings attached to the inner wall of thedrilling collar. Such an array presents an interference filter providinga stop band at a predetermined frequency for longitudinal sound wavespropagating along the walls of a collar. The device exhibits sufficientdamping within the predetermined frequency range as well as goodmechanical strength. The efficiency of an isolator of this typeincreases proportionally to the number of the rings N as well as to theratio M/μ, where M is the mass of a single ring, μ is a mass per unitlength of the collar. Hence, the efficiency of the isolator is verysensitive to even minor changes in outer dimensions of the pipe as wellas to the changes in demands to its wall thickness.

[0041] The attenuation provided by the isolator section is designed tobe minus forty decibels within the frequency range of twelve througheighteen kilohertz. The isolator design satisfy the mechanicalrequirements specified concerning the limitations on inner diameter,outer diameter, minimal cross section area and others.

[0042]FIG. 3 is a partial illustration of an attenuation system 50 for asound tool (not shown) in a drill collar 52 using an array of hangingmass irregularities 54, 56, 58 . . . (may include up to ten elements)mounted on inner wall 60 of drill collar 52. Mass irregularities 54, 56,58, . . . are secured to inner wall 60 by neck pieces 62 which extendout from outer circumference 64, 66, 68, . . . of mass irregularities54, 56, 58, . . . respectively. Neck pieces 62 are smaller both in depthand width than outer circumferences 64, 66, 68, . . . of massirregularities 54, 56, 58, . . . so that mass irregularities 54, 56, 58,. . . are held firmly against inner wall 60, but not so firmly thatacoustic pulses traveling through drill collar 52 are transferredwithout attenuation. In this manner, mass irregularities 54, 56, 58, . .. are held firmly but not tightly.

[0043] In an alternate embodiment, an inner pipe 64 may be provided toprotect array of mass irregularities 54, 56, 58, . . . from mud flow.Inner pipe may be of any material to isolate mass irregularities 54, 56,58 . . . from the mud flow, however, a material that is non-rigid andhas a degree of flexibility is preferred. A material that is less likelyto transfer acoustic pulses toward the receivers is desired.

[0044] The operation of the attenuation filter may be understood by thefollowing discussion. The attenuator section has N mass irregularitiesor elements, each element having the shape of rings or donuts attachedto the inner surface of a pipe at the points x=x_(j), (where j=1, . . .n). The origin of coordinates coincides with the first irregularity,i.e. x₀=0. The mass of a ring j is m_(j). The distance between twoneighboring elements is:

l _(j) =x _(j+1) −x _(j)

[0045] At x>x_(n), an incident longitudinal sound wave of a unitamplitude traveling towards the origin of coordinates may be denoted by

pe ^(−i[k() x−x ^(_(n)) )−ωt]

[0046] where

[0047] k=ω/c is a wavelength constant,

[0048] ω=2πƒ is an angular frequency,

[0049] c= the velocity of sound.

[0050] Due to the presence of an array there exists (at x>x_(n)) areflected wave p_(r)=V_(n)(ω)e^(ik(x−xn)−1 ωt), where V_(n)(ω) is areflection coefficient for an array of n irregularities. In the presentinvention, the dimensions of irregularities are small as compared withthe wave length at a given frequency ω=2π/k. The density p as well aslinear mass of a pipe μ are also of great importance in the attenuation.In the present invention, the mass m_(j) is much greater than μ h_(j),where h_(j)is the length of attachment zone for the mass m_(j). Such anarray presents an interference filter providing a stop band at apredetermined frequency range for longitudinal sound waves propagatingin the walls of a pipe.

[0051] In the solution of a wave equation, the length of a contact zone,Δl, between a ring and an inner wall of a pipe is small as compared tothe wavelength of interest λ. Under these circumstances the propagationof the longitudinal wave can be described by the following differentialequation: $\begin{matrix}{{{{YS}\frac{\partial^{2}u}{\partial x^{2}}} - {\mu \frac{\partial^{2}u}{\partial t^{2}}} - {M_{j}\frac{\partial u^{2}}{\partial t^{2}}{\delta \left( {x - x_{j}} \right)}}} = 0} & (1)\end{matrix}$

[0052] Where:

[0053] Y is the Young's modulus of the pipe material,

[0054] S is the cross section area of the pipe wall,

[0055] u is the displacement,

[0056] μ is the linear mass of the pipe, and

[0057] x is the longitudinal coordinate.

[0058] When considering propagation of a sinusoidal wave, thedisplacement u may be represented by a function of the formu(x)exp(−iωt), where, ω is the angular frequency, The differential waveequation then takes the form: $\begin{matrix}{{{{YS}\frac{\partial^{2}u}{\partial x^{2}}} + {{\mu\omega}^{2}u} + {M_{j}\omega^{2}u\quad {\delta \left( {x - x_{j}} \right)}}} = 0} & (2)\end{matrix}$

[0059] For an array of N mass irregularities, the solution takes theform $\begin{matrix}{{u(x)} = {{A\quad ^{\quad {kx}}} - {\sum\limits_{j = 1}^{N}{b_{j}{G\left( {x - x_{j}} \right)}{u\left( x_{j} \right)}}}}} & (3)\end{matrix}$

[0060] where,

[0061] A is an initial wave amplitude,

[0062] G(x−x_(j))=exp (ix·x−x_(j·))/(2 y s k) is Green function, and

[0063] b_(j)=M_(j)ω is the magnitude of an irregularity.

[0064] Hence the transmission coefficient at a position x that isgreater than x_(n) can be found as:

[0065] T=u(x)/A, which may be expressed in decibels using the usualconversion factor.

[0066] The transmission coefficient of the array may also be obtained byother methods. One such method is an impedance approach, the relativeinput impedance is given by the formula:

Z _(in)=(p/vρc)

[0067] where:

[0068] p=pressure,

[0069] c=velocity of sound in the medium,

[0070] v=vibrational velocity, and

[0071] ρ=density.

[0072] For an array of N elements, the impedance is calculated with thehelp of the following recurrence procedure:${Z_{in}^{j + 1} = {\frac{Z_{in}^{j} - {i\quad {\tan \left( {kl}_{j} \right)}}}{1 - {i\quad Z_{ɛ}^{j}{\tan \left( {kl}_{j} \right)}}} - {i\frac{{km}_{j}}{\mu}}}},{j = 1},2,{\ldots \quad N}$

[0073]FIGS. 4 and 5 illustrate plots of transmission vs. frequency. Theinfluence of the number of elements is illustrated in FIG. 4.Transmission curves are shown for six elements, eight elements and tenelements. The increase in the number of elements only slightly changesthe transmission curve at the borders of the predetermined frequencyband. However, the attenuation values of the transmission curves in themiddle of the frequency band are greatly affected. The period of anarray 1 is important to place the transmission curves at the properfrequency. In the preferred embodiment an optimal value for the spacingbetween elements is 5.12 inches or approximately thirteen centimetersfor the inner and outer diameter used. However, other spacings such asfourteen or twelve centimeters may also be used and provide acceptableresults. The influence of the mass of a single element is illustrated inFIG. 5.

[0074]FIG. 4 illustrates attenuation curves for arrays of ten elements.Each curve is for elements of different weights. A first curve is forten elements, each weighing eight kilograms, the second for elementsweighing eleven kilograms and a third for elements weighing fourteenkilograms. An increase in the mass M results in changing the lowfrequency border. The high frequency border remains essentiallyunchanged. All the transmission curves show that transmission lossexceeds forty decibels within the predetermined frequency band betweentwelve and eighteen kilohertz.

[0075] The calculations were performed for an array of N identicalequally spaced irregularities. Transmission coefficient was calculatedvs. frequency within the frequency range from five to twenty kilohertz.

[0076]FIG. 6 is a graphical representation of the attenuation of apreferred embodiment of the present invention. In the preferredembodiment, ten elements were used with a spacing of thirteencentimeters between elements. Rings of stainless steel were used as massirregularities 54, 56, 58 . . . It can be seen that the arrangement ofthe preferred embodiment provides attenuation of waves in the range ofeight to eighteen kilohertz. By using his system, interference of wavestraveling through the collar of a drilling tool can be greatly reducedand acoustic logging is possible during a drilling operation.

[0077]FIGS. 7a and 7 b show a comparison between the embodimentdiscussed above with respect to FIG. 2 and an alternate embodiment ofthe invention using a different arrangement of attaching the massdiscontinuities to the drill collar. Shown in the upper portion of FIG.7a is a drill collar 152 a to which a mass 154 a is attached by means ofa neck 158 a. This corresponds to the arrangement discussed above withreference to FIG. 2. Shown in the upper portion of FIG. 7b is analternate arrangement in which a mass 154 b is attached to the drillcollar 152 b over substantially the full length of the mass. Shown inthe lower portion of FIG. 7a is a schematic representation of theeffective mass discontinuity 170 a as seen by a propagating wave:typically, such a mass discontinuity provides approximately 6 to 8 dB ofattenuation of the wave. The lower portion of FIG. 7b shows theeffective mass discontinuity 170 b as seen be a propagating wave:effectively, an attenuation of 2-3 dB of attenuation is provided at eachboundary. By an analysis such as discussed above with respect toequations 1-4, the arrangement of FIG. 7b is shown to act as a low passfilter. By suitable choice of the spacing and size of the weights, theeffective cutoff frequency can be made to be around 10 kHz. When used incombination with an electrical bandpass filter (not shown) on the tool,body waves through the drill collar may be effectively attenuated.

[0078]FIGS. 8a-8 c show alternate embodiments of the invention in whichthe isolator comprises a machined cylindrical member. In FIG. 8a, thecylindrical member has an outer diameter of OD and an inner diameter ofID. The inner diameter allows passage of drilling mud. The inside wallif the cylindrical member has recess of length L therein. A body waveencounters regions of different cross sectional areas and massdensities, similar to the embodiments discussed above, resulting inattenuation of body waves.

[0079]FIG. 8b shows an arrangement in which the recess are on theoutside of the isolator whole FIG. 8c shows an arrangement in whichthere are recess on both the outside and the inside of the isolator.

[0080]FIG. 9 shows the results of a finite element (“FE”) simulation ofthe various embodiments shown in FIGS. 8a-8 c. The abscissa is thefrequency and the ordinate is the normalized amplitude of waves passedby the attenuator. Note that the amplitude scale is linear, rather thanbeing in decibels. The curve 301 shows the spectrum for a cylindricalpipe. The curve 303 shows the spectrum for cuts on the inside of thepipe, 305 is for recesses on the inside and outside of the pipe while307 is for recesses on the outside of the pipe. Similar FE simulationshave been carried out for various lengths L of the recesses. Based uponthese simulations, for an OD of 7.09″, in a preferred embodiment of theinvention, a value of L of 3.15″ (8.5 cm) with recesses on both theinside and the outside of the isolator is used.

[0081] The results in FIG. 9 are for a plurality of equally spacedrecesses having the same length and the same depth of the recesses.Other embodiments of the invention use a combinations of sections havingdifferent lengths and different depths of inner and outer recesses.Examples are shown in FIG. 10. Each section 400 may be considered to bea waveguide with an associated pass-band and a reject band determined bythe inner diameter 403 and the outer diameter 401. As may be seen inFIG. 10, each section has an axis parallel to the longitudinal axis 405of the body of the attenuator. By using such a combination of differentinner and outer diameters, a broad range of frequencies may beattenuated. This attenuation is in addition to the attenuation producedby reflections between adjacent sections 400. In the presence ofborehole fluid on the inside and outside of the sections, the waveguidesare “leaky” waveguides that allow energy to propagate into the fluid. Ina preferred embodiment of the invention, the inner diameters range from2″ to 6″ and the outer diameter ranges from 4″ to 10″.

[0082]FIGS. 11 and 12 show an alternate preferred embodiment using anarrangement of mass bodies attached to an external wall of a drillcollar. The effects are similar to those discussed above in reference toFIGS. 7a and 7 b, however the external arrangement offers advantages ofeasier and less expensive fabrication and easier maintenance than massesconnected to the internal wall of the drill collar as describedpreviously. The mass discontinuities shown in FIGS. 11 and 12 areessentially cylindrical rings. The rings may be made of steelor,alternatively, may be made of a more dense material such as tungsten.In FIG. 11, the mass rings 505 a and 505 b have an internal diameter 501which is greater than the external diameter 503 of the drill collar 504and are attached to the drill collar 504 by necks 506 a and 506 b,respectively. As with the mass discontinuity described previously inFIG. 7a, such a mass discontinuity as shown in FIG. 11 providesapproximately 6-8 dB of attenuation of a direct acoustic wave travelingin the drill collar 504. Note that for simplicity, only two rings areshown in each of FIG. 11 and FIG. 12, however the number of rings willtypically be between 6 and 10 with a spacing between approximately 12and 14 cm. for a frequency range of interest of 10 khz to 20 khz. Notethat this is an exemplary range and that other frequency ranges may befiltered by the appropriate selection of mass size, number and spacingas previously described. An advantage of the external arrangement can berealized because attenuation is related to the mass of each ring 506 a,506 b, divided by the mass per unit length of the drill collar, aspreviously discussed. For example, for similar spacing and length of therings as described in FIG. 7a, the external rings 506 a, 506 b can havea smaller thickness t due to the d² effect on ring volume. Because therings 506 a, 506 b are at a larger diameter than the internal ringdescribed in FIG. 7a, if the length of the rings is the same, rings 506a, 506 b will be thinner to have the same mass for the same material.Alternatively, if the ring thickness t and the length are held the sameas before, then the mass of rings 506 a, 506 b would be greater than themass of the ring of FIG. 7a. The increased mass will result in increasedattenuation for the configuration of FIG. 11 as compared to theconfiguration of FIG. 7a.

[0083]FIG. 12 shows an asymmetrical arrangement for a mass ring attachedto a collar. Exemplary mass rings 605 a, 605 b are coupled to collar 604at shoulder 607 having a raised diameter 606. The masses 605 a, 605 bcontact the collar over a portion K of the length L of the masses 605 a,605 b such that the masses are supported over a portion of their lengthand cantilevered for a portion of their length. The masses may beattached by welding, brazing, press fitting, shrink fitting or any othersuitable technique. For exemplary purposes, the number of masses and thespacing of the masses are essentially the same as for those describedfor FIG. 11. The acoustic source is located in the direction of thesupported portion of the masses 605 a, 605 b, typically an upholedirection, as shown in FIG. 12. As acoustic waves from the source traveltoward the receiver, or downhole, they encounter a geometry which allowsthe acoustic wave to enter the masses 605 a, 605 b and be essentiallytrapped in the cantilevered section. Waves traveling in the oppositedirection do not encounter the same geometry but essentially see onlythe supported section of masses 605 a, 605 b and are not attenuated asmuch as downward travelling waves. The arrangement shown in FIG. 12 ispreferred for a drilling operation because it provides increased supportarea for the masses as compared to that of FIG. 11, thereby providingincreased stability of the masses as they encounter the significant wallforces involved in downhole drilling. The external arrangement of themasses of FIG. 11 and FIG. 12 provide improved cleaning, inspection, andmaintenance compared to the internal mass arrangements describedpreviously. While the masses shown in FIG. 11 and FIG. 12 have sharpcorners, radiused corners may be provided for stress relief and/or tofacilitate ease of manufacturing. Such techniques are known in the artand are not described further.

[0084] While there has been illustrated and described a particularembodiment of the present invention, it will be appreciated thatnumerous changes and modifications will occur to those skilled in theart, and it is intended in the appended claims to cover all thosechanges and modifications which fall within the true spirit and scope ofthe present invention.

1. A system for attenuation of acoustic waves traveling through alongitudinal member capable of transmitting said acoustic wavestherethrough comprising: a plurality of spaced-apart masses firmlyattached to an adjacent outer wall of said longitudinal member, eachsaid plurality of masses having a predetermined spacing and apredetermined magnitude for attenuation of acoustic pulses in apredetermined frequency range.
 2. The system for attenuation of acousticwaves according to claim 1 wherein said predetermined frequency rangecomprises 10 khz to 20 khz.
 3. The system for attenuation of acousticwaves according to claim 2 wherein said plurality of masses comprises amaterial selected from (i) steel rings, and, (ii) tungsten rings.
 4. Thesystem for attenuation of acoustic waves according to claim 3 whereinsaid plurality of masses is between six and ten.
 5. The system accordingto claim 1 wherein said spacing of the masses is within the range oftwelve to fourteen centimeters.
 6. The system according to claim 1wherein the masses comprise metal rings attached to the outer wall ofthe longitudinal member by neck pieces extending inward from an innercircumference of the rings.
 7. The system according to claim 1 whereineach of said plurality of masses is attached to the longitudinal memberby at least one neck piece.
 8. The system according to claim 1 whereinthe masses comprise metal rings attached to a shoulder on thelongitudinal member.
 9. The system according to claim 8 wherein themetal rings are asymmetrically attached to the shoulder on thelongitudinal member.
 10. An apparatus for performing acousticinvestigations of a subsurface geological formation penetrated by aborehole comprising: (a) a longitudinally extending body conveyed insaid borehole; (b) an acoustic transmitter supported by the body, saidtransmitter generating acoustic signals in the body, the borehole andthe subsurface formations; (c) an acoustic receiver spaced apart fromthe transmitter and supported by the body for receiving said acousticsignals; and (d) an attenuator located on a substantially cylindricalportion of the body having an inner diameter and an outer diameter,between said acoustic transmitter and said acoustic receiver forattenuating said acoustic signals in the body within a predeterminedfrequency range; wherein said attenuator comprises a plurality ofspaced-apart masses having a predetermined spacing, mass and lengthfirmly attached to an outer wall of the cylindrical portion of the body.11. The apparatus of claim 10 wherein the longitudinally extending bodyis conveyed on a drilling tubular having a drillbit therein for drillingthe borehole, said drilling tubular selected from the group consistingof (i) a drillstring, and, (ii) coiled tubing.
 12. The apparatus ofclaim 10 wherein the attenuator comprises a plurality of spaced apartmasses wherein said predetermined frequency range comprises 10 khz to 20khz.
 13. The apparatus of claim 10 wherein the attenuator comprises aplurality of spaced apart masses wherein material of said masses isselected from the group consisting of (i) steel rings, and, (ii)tungsten rings.
 14. The apparatus of claim 10 wherein the attenuatorcomprises a plurality of spaced apart masses wherein said plurality ofmasses is between six and ten.
 15. The apparatus of claim 10 wherein theattenuator comprises a plurality of spaced apart masses and wherein saidspacing of the masses is within the range of twelve to fourteencentimeters.
 16. A method of performing acoustic investigations of asubsurface geological formation penetrated by a borehole comprising: (a)conveying a logging tool having a substantially cylindrical body intothe borehole; (b) activating a transmitter on the body for generatingacoustic signals in the formation, borehole and the body; (c)attenuating signals passing through the body using an attenuatorcomprising a plurality of spaced-apart masses firmly attached on anoutside adjacent wall of the body, said masses being spaced apart apreselected distance to attenuate signals within a specified frequency(d) using a receiver on a side of the attenuator opposite thetransmitter for receiving signals through the formation and theattenuated signals through the body.
 17. The method of claim 16 whereinsaid specified frequency range comprises 10 khz to 20 khz.
 18. Themethod of claim 16 wherein said plurality of masses comprises a materialselected from (i) steel rings, and, (ii) tungsten rings.
 19. The methodof claim 16 further comprising conveying the logging tool on a drillingtubular.
 20. The method of claim 16 further comprising performing saidacoustic investigations during drilling of the wellbore.
 21. A systemfor attenuation of acoustic waves traveling through a longitudinalmember capable of transmitting said acoustic waves therethrough,comprising a plurality of spaced-apart masses firmly and asymmetricallyattached to an adjacent outer wall of said longitudinal member, eachsaid plurality of masses having a predetermined spacing and apredetermined magnitude for attenuation of acoustic pulses in apredetermined frequency range.
 22. The system according to claim 21wherein the plurality of masses comprises a material selected from (i)steel rings, and (ii) tungsten rings.
 23. The system according to claim21 wherein the predetermined frequency range comprises 10 khz to 20 khz.24. The system for attenuation of acoustic waves according to claim 21wherein said plurality of masses is between six and ten.
 25. The systemaccording to claim 21 wherein said spacing of the masses is within therange of twelve to fourteen centimeters.
 26. A method of performingacoustic investigations of a subsurface geological formation penetratedby a borehole comprising: (a) conveying a logging tool having asubstantially cylindrical body into the borehole; (b) activating atransmitter on the body for generating acoustic signals in theformation, borehole and the body; (c) preferentially attenuating signalspassing through the body in a predetermined direction using anattenuator comprising a plurality of spaced-apart masses firmly andasymmetrically attached on an outside adjacent wall of the body, saidmasses being spaced apart a preselected distance to attenuate signalswithin a specified frequency range; (d) using a receiver on a side ofthe attenuator opposite the transmitter for receiving signals throughthe formation and the attenuated signals through the body.
 27. Themethod of claim 26 wherein said specified frequency range comprises 10khz to 20 khz.
 28. The method of claim 26 wherein said plurality ofmasses comprises a material selected from (i) steel rings, and, (ii)tungsten rings.
 29. The method of claim 26 further comprising conveyingthe logging tool on a drilling tubular.
 30. The method of claim 26further comprising performing said acoustic investigations duringdrilling of the wellbore.